When deciding on a testing system to test your materials and products, you might come across materials testing systems that offer an open-loop controller and others that offer closed-loop controller feedback. While you may already know these terms from other contexts, this blog post will guide you through a comparison of open-loop and closed-loop controls in the context of materials testing.
Principles of Operation
Universal testing systems with open-loop and closed-loop systems both work with a controller that operates the machine’s movement. The controller sends information to the motor (e.g. to move the machine at 20mm per min). Once the input passes into the start of the system and produces the output, the motor initiates the movement of the crosshead or the ram accordingly.
The main difference between an open-loop system and a closed-loop system is that the closed-loop system has the ability to self-correct while the open-loop system doesn’t. Consequently, closed-loop systems are often called feedback control systems while open-loop systems are also known as non-feedback controls.
Open-Loop Test Systems
Open-loop systems tend to be simple and inexpensive as they do not provide feedback from the machine movement to the controller. In other words, open-loop systems act solely on the basis of the input and do not use feedback from the output to self-correct while the test is running. Thus, the test procedure entered into an open-loop controller may vary due to external disturbances, like noise, without the operator noticing. The data acquired from a universal testing machine with an open-loop controller may be imprecise.
In materials testing, when running systems with an open-loop controller, keeping the speed constant might become a problem as there is no direct feedback to monitor and regulate control. The speed of the machine might change during a test due to various reasons, and the open loop system does not have a feedback control to let the controller know of these changes. The test will run at a modified speed without the machine or the operator noticing. This is a problem especially if tests are to be run per ASTM standards or in any test where the speed has to be kept constant.
Closed-Loop Test Systems
Universal testing machines with closed-loop control systems have a feedback loop that continuously sends information from the closed-loop controller to the motor and also from the motor to the closed-loop controller. This constant feedback allows certain variables such as the load rate and the stress rate to remain as specified throughout the tests. Closed-loop systems provide higher accuracy due to the ability to react immediately to possible changes.
Below is a diagram showing how closed-loop testing systems operate. All ADMET testing equipment comes with a closed loop-controller.
Diagram of a closed-loop system